When properly designed and implemented, automobile air bag restraint devices can dramatically reduce the injury and loss of life caused by automobile collisions. When a collision occurs, unrestrained occupants of the colliding car are often thrown forward against the interior walls and furnishings of the car. Unrestrained occupants may also be thrown through the windshield, out of the car, and onto the road or other surroundings. In either case, the occupants may be seriously injured or killed.
An air bag properly deployed in front of each occupant may cushion or prevent impact of the occupant against anything except the cushioning of the bag. Proper air bag deployment includes inflating the air bag to a volume sufficient to cushion the occupant. The inflation must be rapid, so that the inflated air bag is in place in time to protect the occupant. The air bag should also deflate while absorbing the occupant's momentum, so that energy is expended through the deflation and the occupant does not simply rebound from the air bag. By absorbing kinetic energy from car occupants during a collision, air bags may reduce or prevent the injuries and loss of life caused by the collision.
An air bag, also known herein as a "gas bag," includes a gas generant located inside a container. When a collision occurs, an igniter is used to initiate combustion of the gas generant. As the gas generant combusts, it produces gases which increase both the temperature and pressure within the container. The increased pressure inflates a bag which is attached to the container. Thus, one performance goal in designing gas bag inflators is to provide a gas generant and a container which inflate the gas bag to a sufficient volume to cushion an occupant during collision.
Another performance goal in designing gas bag inflators is to provide an igniter, gas generant, and container which provide sufficient pressure quickly enough to be of use in cushioning car occupants. An occupant of a car traveling at 55 miles per hour is traveling at about 80 feet per second. When the car slows or stops suddenly upon collision with another object, the unrestrained occupant continues traveling forward, relative to the car, at speeds up to about 80 feet per second. Thus, if the inflated air bag is to be in place about one foot in front of the occupant's position upon collision, the bag must be inflated in about 1/80th of a second (12.5 milliseconds) after the collision.
A third performance goal in designing gas bag inflators is to provide a gas generant and container which supply sufficiently cool inflation gas to the gas bag. Known gas generants combust at temperatures as high as 4600 degrees Fahrenheit. The air bag being inflated typically is not made of materials that are good thermal insulators. Moreover, the air bag is ordinarily thin enough to pack in a folded position in the car's steering column or dashboard. Because the air bag itself does not insulate the occupant from the hot gases inside the bag when the bag is inflated, the gas temperature must be reduced to prevent the occupant from being burned by contact with the bag. If the inflation gas is too hot, it may also burn through the air bag, thereby reducing the bag's effectiveness. Some of the measures that may be taken to reduce temperature inside the inflated gas include varying the composition of the gas generant, including heat sinks in the container, altering mass flow rates within the container, and providing part or all of the container with gases other than those produced by combustion of the gas generant.
In addition to providing the bag with an acceptable pressure and temperature at an acceptable time, gas bag inflators must also pursue other goals. The gas bag inflator's size, weight, cost, reliability, stability, and other characteristics are also important. Designing and evaluating a good gas bag inflator therefore requires consideration of numerous factors, and many different designs are often considered before a design meeting predetermined performance goals is obtained.
One approach to evaluating different gas bag inflator designs is to fabricate and actuate a test inflator and measure the pressure, temperature, and other gas dynamic and heat transfer characteristics of interest. At a minimum, such fabrication testing requires mixing a quantity of gas generant, making or modifying a container, placing the gas generant in the container, placing measuring devices such as thermocouples and pressure sensors at appropriate locations in or near the container, combusting the gas generant, and reading measurements on the measuring devices.
Because it involves engineers and technicians from several different disciplines, fabrication testing is time-consuming and labor-intensive. Chemists, gas dynamicists, metallurgists, and experts from several other fields must coordinate their efforts for each fabrication test. Obtaining all the necessary parts and making all the necessary preparations typically takes much longer than the actual fabrication test and the evaluation of the test results.
Fabrication testing is also expensive because the chemical compounds, containers, and measuring equipment employed are subject to wear or destruction during each test. In general, a new quantity of the igniter composition and a new quantity of the gas generant composition must be provided for each fabrication test. Although a given container may be reusable after being cleaned, portions of the container which absorb substantial heat or which receive chemical deposits from the inflation gas mixture may need replacement after one or more fabrication tests. Moreover, the container dimensions may need to be altered from one fabrication test to the next. The measuring equipment used in fabrication tests is also subject to pressure and temperature extremes that require periodic maintenance or replacement of sensors.
Thus, it would be an advancement in the art to provide a system and method for evaluating gas bag inflators which reduces the need for fabrication testing.
It would also be an advancement in the art to provide such a system and method which predict the temperature and pressure performance of a gas bag inflator without destructive testing of gas generant compounds.
It would be an additional advancement to provide such a system and method which assist in the design of gas generants and gas generant containers to meet specified performance goals.
It would be a further advancement in the art to provide such a system and method which can be tailored to consider selected gas dynamic and heat transfer characteristics of a gas bag inflator while ignoring other characteristics.
Such a system and method are disclosed and claimed herein.